A turbomachine, such as a steam or gas turbine is driven by a hot working gas flowing between rotor blades arranged along the circumference of a rotor so as to form an annular blade arrangement, and energy is transmitted from the hot working gas to a rotor shaft through the rotor blades. As the capacity of electric power plants increases, the volume of flow through industrial turbine engines has increased more and more and the operating conditions (e.g., operating temperature and pressure) have become increasingly severe. Further, the rotor blades have increased in size to harness more of the energy in the working gas to improve efficiency. A result of all the above is an increased level of stresses (such as thermal, vibratory, bending, centrifugal, contact and torsional) to which the rotor blades are subjected.
In order to limit vibrational stresses in the blades, various structures may be provided to the blades to form a cooperating structure between blades that serves to dampen the vibrations generated during rotation of the rotor. For example, mid-span snubbers, such as cylindrical standoffs, may be provided extending from mid-span locations on the blades for engagement with each other. Two mid-span snubbers are located at the same height on either side of a blade with their respective contact surfaces pointing opposite directions. The snubber contact surfaces on adjacent blades are separated by a small gap when the blades are stationary. However, when the blades rotate at full load and untwist under the effect of the centrifugal forces, snubber surfaces on adjacent blades come in contact with each other. In addition, each turbine blade may be provided with an outer shroud located at an outer edge of the blade and having front and rear shroud contact surfaces that move into contact with each other as the rotor begins to rotate. The engagement between the blades at the front and rear shroud contact surfaces and at the snubber contact surfaces is designed to improve the strength of the blades under the tremendous centrifugal forces, and further operates to dampen vibrations by friction at the contacting snubber surfaces. A disadvantage of snubber damping is that on large diameter blades it is often difficult to achieve the desired contact forces produced between snubbers as a result of the centrifugal untwisting of the blades. In addition, the large mechanical load associated with large diameter blades typically necessitates larger snubber structures for mechanical stability to avoid outward bending of the snubber, resulting in increased aerodynamic losses and flow inefficiencies due to the flow restriction of larger snubbers positioned in the high velocity flow area through the part-span area.